vfio/container: Change VFIOContainerBase to use QOM

vfio/container: Change VFIOContainerBase to use QOM

VFIOContainerBase was made a QOM interface because we believed that a
QOM object would expose all the IOMMU backends to the QEMU machine and
human interface. This only applies to user creatable devices or objects.

Change the VFIOContainerBase nature from interface to object and make
the necessary adjustments in the VFIO_IOMMU hierarchy.

Reviewed-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Tested-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>